The present invention concerns a thread joint for steel pipes, usable in particular in the petroleum industry, which comprises a sealing device located at the level of the threadings.
It is known that joints for pipes used in the petroleum industry must present a certain number of characteristics that are difficult to reconcile and which consist in particular of the fact that the joint while being easy to machine and not very sensitive to damage, permits ensurance of perfect sealing with respect to very strong pressures that are exerted either on the inside or outside of the joint, even under the hypothesis where the joint is subjected to bending or substantial tension or axial compression.
It is also necessary that the elements of the joint be interchangeable without having an unfavorable effect on the quality of sealing obtained during new screwing of the joint with other elements. The threaded joints now used in the petroleum industry mostly comprise male and female threadings that are most often conical (but which can also be cylindrical) and which ensure mechanical maintenance of the two elements of the pipe, one within other, essentially supporting the axial tensions exerted on the joint.
In most cases the known joints also comprise so-called screw stops whose purpose is to immobilize the male element relative to the female element as soon as the prescribed tightening torque has been reached.
Finally, the joints of this type also contain at least one pair of sealing surfaces that are placed so that during tightening of the joint these sealing surfaces come in contact with each by being applied to each other by a sufficient pressure to obtain sealing.
In most cases this is achieved by contact of the metal-metal type, but in certain cases the sealing surfaces can be associated with joints made of plastic, such as PTFE rings which by deformation ensure or contribute to sealing of the joint.
It is known in particular that such annular joints, for example, made of PTFE, can be placed in a groove made on the female element, or at the level of the sealing surfaces, or even at the level of the threadings so that during tightening of the joint the male element causes plastic deformation of the joint which loses its initial shape and adapts itself to the shape of the male element.
Such plastic joints, however, present the drawback of being difficult to emplace and in practice require changing each time the joint is disassembled and then reassembled.
Such joints consisting of plastic rings that are simply positioned in a groove made in the female element must in effect be very strongly deformed in order for the male element to inlay its threadings.
In practice this causes destruction of the plastic joint, which is thus unusable for later installation of joint elements. Moreover, the substantial stresses imposed on the plastic joint during this deformation often cause displacement of the plastic ring which is thus extracted from the groove and no longer functions in its role.
It is also known, in particular for achieving sealing of screwed joints intended to supply buildings with water or central heat, that there is a technique consisting of covering the threading of one of the elements by means of a thermoplastic material that is deformed during screwing to ensure tightness.
These sealing devices are intended to replace the use of flax or PTFE strip which are conventionally wound onto the threading of the male element before screwing into the female element. These sealing device seek to preposition a thermoplastic material at the level of the threading of these elements.
In this case as well it is generally not possible to lossen the joint, to retighten it by interchanging the elements, preserving the quality of tightness required.
The present invention seeks to impart to joints for steel pipes, intended particularly for the petroleum industry, satisfactory sealing at the level of the threadings, the sealing being achieved by a plastic element whichis an integral part of one of the elements of the joint, which can be reused reliably during subsequent tightenings of this element.
It is known that in order to permit its original function, which is to mechanically connect the two elements of the pipes, each thread of the male element must be applied to a corresponding thread of the female element necessarily in the axial direction and most often (in the case of truncated threadings) in the radial direction as well.
It follows that one can break down the contour of the threads of one joint in the clamped condition, on the one hand, into surfaces that are strongly applied to each other during screwing of the joint, and on the other hand, into surfaces of the male element and female element that are opposite each other at a certain distance from each and between which the play existing in the threading is located.
It follows from the existence of this part of the contour of the threadings where the surfaces ofthe male and female elements are separated from each other, that to date it has not been possible in practice to achieve satisfactory sealing at the level of the threadings of this type of joint.
In effect, the existence of this play between the corresponding surfaces of the male threading and female threading creates a leakage path with essentially helicoidal shape along the threadings, which opposes any sealing at this level.
The present invention is based on the achievement of sealing which is located along this helicoidal leakage path which can thus be blocked effectively and repeatedly.